Heretofore, in order to weld blades of an impeller having a narrow discharge width to a shroud plate, which must be performed within a confined space or a space to which it is difficult to gain access due to a limited small space, arc welding using a coated electrode has been generally employed. However, the surface of the bead according to this welding is not smooth, but is rather rough, thereby producing undesirable notch sensitivity at the toes of the weld bead. In order to improve the fatigue strength in such case, it is necessary to trim or cut the toes of the weld bead and to smooth the surface of the bead. Further, in arc welding using a coated welding wire, spattering is likely, and the spatter produced must also be removed. However, since the width of discharge port of the impeller is narrow, not only the welding operation itself but also the post-welding operations such as trimming and finishing the rough surface are difficult and consume many man hours.
To obviate such drawbacks, automatic welding with NC-control has been considered, but this increases the equipment cost and the maintenance thereof is also expensive. Further, the configuration of the line of weld varies in each instance due to the manufacturing tolerances of the components parts such as blades which may cause difficulty in applying NC-control to such a welding operation.
MIG-welding has also been considered for welding such construction. However, the welding torch is generally cooled by water so a relatively large size cooling chamber is required to make a suitable torch; thus, this process is usually unsuitable for welding an impeller. Further, the surface of the bead produced by MIG-welding is not smooth and a smoothing operation is required.
Submerged arc welding provides satisfactory results under certain conditions. However, it is difficult to obtain a satisfactory bead unless a high current is used. Also, supplying flux to a curved area is difficult.